Semiconductor lasers arc widely used and have reached a high state of development. For instance, lasers have been developed that can have relatively narrow linewidths. Such lasers are potentially important for, e.g., coherent optical communication systems. Semiconductor lasers arc typically grown by some vapor deposition technique, e.g., by metal organic vapor phase epitaxy (MOVPE).
Lasers that comprise distributed feedback means (such lasers are usually referred to as DFB lasers) have the potential of achieving very narrow linewidths, since the radiation reflected from the feedback means into the laser cavity can have a very narrow frequency range. The distributed feedback means typically comprise a "grating", exemplarily a regular sequence of elevated and depressed regions in the surface of a semiconductor layer, overgrown with material of different refractive index (exemplarily quaternary material, e.g., InGaAsP), with the interface sufficiently close to the active region of the laser such that the lasing optical field can interact with the grating.
An important substrate material for semiconductor lasers is InP, since it can be used to grow lasers that emit radiation in the approximate 1.3-1.55 .mu.m wavelength region, known to be particularly advantageous for optical fiber communication systems, due to the low loss of SiO.sub.2 -based optical fiber at some wavelengths in that regime. In InP-based DFB lasers the gratings can be etched in InP or in an InGaAsP layer previously grown on an InP substrate. The gratings typically are produced by photolithography, followed either by wet chemical etching or by reactive ion etching (RIE).
It has often been observed that during growth of a semiconductor material on a grating, the geometrical dimensions (e.g., grating amplitude, duty cycle, or profile) of the grating are changed. The degree of change depends upon the details of the cleaning and/or heat treatment of the grating prior to the growth and also on the method used to grow the overlying semiconductor material. There are various growth methods commonly used; e.g., Liquid Phase Epitaxy (LPE), Metal-Organic Vapor Phase Epitaxy (MOVPE) or Hydride Vapor Phase Epitaxy (HVPE).
Since laser characteristics typically depend significantly on the geometry of the grating, the observed changes can be highly undesirable, especially if the changes are difficult to control. Thus, it would be beneficial to have available a method of making InP-based DFB lasers that can be substantially free (or is at least less subject to) changes in grating geometry. This application discloses such a method.